So far, a natural leather, a leather made from vinyl chloride, a synthetic leather or an artificial leather are widely used for various uses, and in many cases, a product is made by adhering or sewing up such plural leather panels.
Such leather products are required to be imparted with water proof properties in accordance with its use. For example, since in the case of a sport goods such as a pair of sport shoes, a ball for a ball game, etc., the products are sometimes used in a rainy weather, research works have been carried out for reinforcing the water proof properties of these leather products such as applying a water repelling treatment to the leather panels, etc. in order to improve the water proof properties of these leather products.
However, although the improvement of the water proof properties of the leather panel itself is carried out to a certain extent, the fact that the penetration of water into the inside of the products through seams produced by sewing up theses leather panels, remains as a problem.
Then, as to the waterproofness of the seams in the leather products, the present inventors have carried out various research works for preventing the penetration of water into the inside of the leather products by applying a water absorbing sewing thread used for the water proof clothings such as a raincoat, but there was no way of satisfying the waterproofness and the strength of the seams in the leather products receiving a severe shock such as the sport goods.
As for the above water absorbing sewing thread, the following materials are cited. For example, in the specification of Japanese Utility Model Publication No. 63-166170, in order to prepare a pair of boots by sewing a vinyl sheet backlined with a urethane foam, it was proposed to use a fiber produced by twisting an acrylic water absorbing polymer filament around the outside of an acrylic fiber (Lanseal F; Registered Trade Mark, produced by Nippon Exlan Industries, Co., Ltd.) as the sewing thread. In this sewing thread, its outer layer swells by absorbing water to close the seam for aiming at the prevention of the penetration of water from the seam. However, the sewn product produced by using the above sewing thread has insufficient swelling rate of the used thread and strength of the seam, and this method is not suitable for sewing the leather products, especially for the sport leather products.
Also, in the specification of JP-A 6-185033 (hereinafter, JP-A means "Japanese Unexamined Patent Publication"), similarly to the Japanese Utility Model Publication No. 63-166170, it was described to use an ultra water absorbing fiber consisting mainly of an acrylic polymer (Lanseal F; Registered Trade Mark, produced by Toyobo Co., Ltd.) for sewing up two water blocking sheets (prepared from the ultra water absorbing fibers). However, the sewn product produced by using the above sewing thread also has insufficient swelling rate of the used thread and strength of the seam, and this method is not suitable for sewing the leather products, especially for the sport leather products.
Also, in the specification of JP-A 58-174646, a thread obtained by treating an acrylic fiber with an alkali as a machine thread for a water proof products such as a raincoat, an anorak, a tent, etc., was described. But the machine thread is also not suitable for the sewing thread for the leather products, especially for the sewing thread for the sport leather products similarly to the above.
Further, in the specification of JP-A 58-156081, it was described to sew up a water proof cloths such as a raincoat, an anorak, a tent, etc., by using a water proof thread obtained by attaching 5-30 weight % of a low melting resin powder having 150 .mu.m or less particle diameter thereto. This water proof thread prevents water by closing needle holes by heat-treating the water proof cloth after sewing up the cloth, at a temperature of a melting point of the resin powder. The sewn product produced by using this thread is not suitable for the waterproofing of the leather products, especially for the sport leather products, since the molten resin has hardened at the seam parts and can not deal with a deformation of the seams occurred on receiving a shock, etc., in use thereof, and water penetrates from a crack formed by the shock.
In the specification of JP-A 7-290577, it was described to sew up a product prepared by laminating a water swelling intermediate sheet and a fiber sheet such as a woven, knitted or nonwoven fabric, etc., between two water blocking outside sheets made from a common rubber or a synthetic resin in surface and rear by using a water absorbing sewing thread containing water absorbing fibers having a water swelling and water absorbing layer around a core fiber. However, even by sewing with this water absorbing sewing thread, the effect of closing the seam is insufficient and thids method is insufficient for waterproofing the leather products, especially for the sport leather products.
Thus, a soccer ball prepared by using a conventionally used water absorbing sewing thread, has a defect that the seams of artificial leather panels covering the surface of the soccer ball deform on using the same in a rainy weather, a lot of water penetrates through its crack, and is not only absorbed by a sewing thread having a water absorbing property, but also absorbed by nonwoven fabrics constituting the artificial leather to increase the weight of the ball. And also, at the same time, since it receives a severe shock and the seams get a heavy load more than those of the clothing products, it is required that the sewing thread itself has a sufficient strength.
In this way, since the leather products, especially the sport leather products, more specifically the balls for the ball games, and above all the soccer balls are used in a rainy weather and receive a severe shock, the balls less in penetration of water into the inside of the ball and having the seams of a high strength were required.
Then, in order to solve the defects in these conventional technologies, the object of this invention by the present inventors, is to provide the sewing thread for the leather products excellent in water absorption and strength of the seam. Further, another object of the invention is to provide the leather product excellent in strength of the seams and also a water proof properties, especially having a low dynamic water absorption rate for not losing the water proof properties even in the case of giving a shock thereto, by using the above sewing thread for the leather products.